Depends on the organization & specific project context.
At a minimum, I'd expect your sponsor and customer to sign-off assuming the PM is the one "holding the pen" on the PM Plan. But there might be other key stakeholders (e.g. functional managers, solution leads) who also are required to sign-off based on a given organization's standards.
In any Organization , Project Manager along with PMO shall approve the Project Management Plan and submit to Client / Customer for final approvals. Saving Changes...
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